Francisella tularensis is an extremely virulent Gram negative, facultative intracellular bacterial pathogen capable of causing rapidly progressing lethal infections in literally hundreds of diverse animal species, including humans. Although the general properties of Francisella virulence and pathogenesis are understood, the function of any one Francisella protein not assured by sequence homology remains essentially unknown. This situation is due, in part, to the fact that most open reading frames in Francisella have no known orthologs. Our goal is to uncover and experimentally determine unknown mechanisms and properties that contribute to virulence and pathogenesis. Herein we propose to define the function of RipA, which is a cytoplasmic membrane protein that is conserved among Francisella and is required for adaptation to the host cell environment. This goal will be accomplished through genetic and phenotypic analysis of ripA mutants and extragenic suppressors of ripA mutant phenotypes, identification of RipA - binding proteins, biophysical studies on RipA topology and structure, and regulation of ripA expression.